Dew point hygrometer



Oct. 24, 1950 F. A. FRISWOLD ETAL Re. 23,287

DEW POINT HYGROMETER Original Filed Sept. 25, 1945 Frank A. Friswa/dRalph D. Lewis seams a; 24, 1950 DEW POINT HYGBOMETIB Frank A.l'riswold, Lakewood, and Ralph D. Lewis. Parma, Ohio Original No.2,486,696, April 12, 1949, Serial No. 818,604, September 25, 1945.Application for rel-be May 12, 1950, Serial No. 163,802

Claims. (Cl. 73-17) (Granted under theact of March a, 1883, as amendedApril :0, ms; 370 o. o. 751) Matter enclosed in heavy brackets II IIaliv in the riginal patent but forms no matter printed In italicsindicates the additions made by reissue reissue specification;

This invention relates to hygrometer apparatus and in particular to suchapparatus of the dew-point type.

The general object oi the invention is to provide a hygrometer oi thetype referred to which 5 is more stable and reliable 'in operation.

Another object is to provide a dew-point hygrometer in which thermal lagis reduced to a negligible factor, thereby improving its responsecharacteristic to rapidly changing variations in i0 humidity as is foundin aeroplanes flying at high speeds through clouds, etc.

' Another object is to provide a mirror type, dew-point hygrometerutilizing a novel photronic relay in which the average heater currentfor the mirror is not ailected by changes in intensity of the lightsource associated with the photo-cell elements or by changes inoperating voltages of the power supply source.

These and other objects of the invention will become more apparent fromthe detailed description to follow when considered with the accompanyingdrawings which illustrate a preferred embodiment of the invention.

In the drawings,

Fig. l is a schematic wiring diagram oi the electrical circuits of ourimproved hygrometer; and

Fig. 2 is a longitudinal section showing the relationship between thevarious operating components of the apparatus and illustrating thecircuit components in block form. I

Dew-point type hygrometers using a cold mirror to which heat is applieduntil the moisture thereon is evaporated and controlling the heatercurrent for the mirror as aiunction of the amount of light from a lightsource which falls upon a photo-cell unit after reflection from themirror are not broadly new. However, one disadvantage of this type ofhygrometer prior to our improvement was that variations in intensity ofthe light source caused considerable changes in the temperature of themirror thus introducing an error into the hygrometer, since humidity isdetermined as a function of the temperature of the mirror, Inasmuch asillumination varies as the eighth power of the filament current in anelectric lamp, it is obvious that lack of proper current regulation canintroduce serious errors. Aging efiects in the lamp also arecontributing factors.

Another disadvantage of hygrometer construction prior to ourimprovements was that the mirror temperature was subject to aconsiderable amount of hunting. The source of heat the bronze sheetingi5.

part of this was a. resistor imbedded in the mirror head through whichthe heating current flowed. -Be-' cause of the thermal lag or the mirrorhead and the large quantities of heat involved, a serious hunting en'ecttook place when the dew-point changed. It was not unusual to observe alive minute lag period during which equilibrium conditions were beingre-established. This hunt- 1 ing effect made the determination of thedewpoint diflicult, especially under operating conditions where it issought to record rapidly changing humidity conditions such as isexperienced in aircraft flight.

From the description 0! our improved hygrometer construction which'nowfollows, it will become apparent that the above referred todisadvantages are all overcome. Referring now to the drawings, there isincluded a box III of Bakelite divided into three sections; a coolantcompartment or heat sink H, a mirror compartment II, and a photo-cellcompartment ii.

To the Bakelite walls III or coolant compartment II are secured innerwalls ll of insulation material such as balsa which are lined withbronze sheeting IS. The coolant compartment I l is covered with aclose-fitting lid also of Bakelite and balsa'lined with sheet bronze.This lid is not shown because the details of construction of box ID donot constitute the invention claimed in this application. A copper rodl6 0! diameter runs the length of the compartment ll near the bottom andis soldered at each end to A smaller copper rod I I of diameter is setin a hole provided in one end of rod l6 and extends through the balsawall II to the mirror compartment l2. When the hygrometer is in use,compartment II is filled with CO2 l8 or any other suitable type ofrefrigerant, Rods l6, l1 and the CO: l8 thus constitute a heat sink orheat absorber for pulling down, by conduction, the temperature of themirror element. The latter isconstituted by a nickel cap II of /4"diameter fitted on the end of the small rod l1, and which is polished onone face 22 to act as the mirror. Nickel is preferred as the materialfor the cap ll in view of its favorable resistivity, thermalconductivity, magnetic characteristics, and its ability to take a highpolish.

Cap II has a very small hole drilled into it from the side just beneaththe surface 22. -Into this hole there is inserted a 32 gageiron-constantan thermocouple 23.' Thermocouple 23 is connected viaconductors 24, 25 to a. temperature measuring device 20 of conventionalconstruction photo-cell lia and resistor ll.

to thereby measure the temperature of the mirrored face 22 or the capii.

The seams of Bakelite walls iii defining the mirror compartment I! aresealed with Permatex to make the compartment leak-proof. A Lucite window(not shown) may be located near the top oi this compartment to permitobservation of the mirrored cap member 2i. When the instrument is inuse, this window is covered with a Bakelite lid to keep out stray light.Brass tubing connections 21, 28, which extend through opposite walls ofcompartment i2 to the interior thereof, serve as inlet and outlet,respectively, for the air under humidity test.

The photo-cell compartment il contains a type 920 photo-cell ll which iscomprised of two photo-sensitive cell units lla and lib. Also includedin this compartment is a 32-candle power lamp ll and a lens system llsupported within a tubular housing ll that is secured to a wall ll oftransparent plastic such as Lucite for separating compartments I! fromil. Light from lamp ll is focused by lens ll onto the mirrored face 22of cap 2i and is reflected back through wall ll towards the photo-celllib. A cover member l! is provided over the twin photo-cell unit li sothat all direct light from the lamp ll as distinguished from thatreflected from the mirrored face 22, has no eifect on the photo-cel1unit li. However, cover l! is .provided with a slidably mounted door llcooperative with an opening in the cover to permit a small amount oflight to fall on photo-cell lia. This arrangement is desirable to permitbetter balancing of the bridge circuit in which the photo-cells lla andlib are connected. The Lucite wall ll is coveredwith an opaque coatingto except where it is necessary for the direct and reflected light raysto pass through.

The electronic components of our improved hygrometer are shown in thecircuit diagram of Fig. 1. In this diagram, it is seen that photo-cellslia and lib are connected in a Wheatstone. bridge which includes abattery 40 which functions as a power source for the bridge, anotherbattery I which supplies a grid biasing potential for a pair ofamplifier tubes 4!, ll, resistor 44 and resistor ll. One arm of thebridge network consists of photo-cell lib and resistor 45, and the otherbranch of the bridge network is constituted by Initially, thephoto-cells li a and lib are so arranged that with no condensation ofair on the mirrored face 22 of cap 2i, the beam of light from the lampll when reflected by the mirrored face 22 will pass by the edge ofphoto-cell lib along path a. Con tact arm 48 is then adjusted until thevoltage drop across resistance 44 is equal to the drop across theresistance 45. Door ll is adiustable to balance the amount of straylight falling on thephoto-cells lla and li-b. Under these conditions, novoltage will appear across the neutral points 41, I of the bridgenetwork. and the latter is therefore in balance. With the bridge now ina balanced state. variation in intensity of the light from source Aconductor i leads from point 41 to the control grid 42a of tube 42, andconductor I2 leads from point 48 on the brid e to-control grid lla oftube 43. The cathodes 42b and llb of the tubes 42 and I3, respectively,are tied together and connected to the negative terminal of battery ii.It is thus seen that resistor 44 is connected in the cathode-gridcircuit of tube 42, and the rell will have no effect upon the accuracyof operation of the apparatus.

single :loop l4 around -.-plate sistor II is similarly connected in thecathodesrid circuit of tube ll.

The anodes flc and llc of tubes 42. ll are connected to opposite ends ofa load resistor ll. This latter resistor is connected in series with avariable grid biasing potentiometer ll to the cathode-grid circuit oftube ll, the cathode being indicated by a and the grid by llb. The anode"c of tube II is connected to a plate resistor II, and the circuitelements are such that maximum current flows in the anode-cathodecircuit of tube 55, and hence also through resistor 56, when no lightfrom the lamp ll as reflected by the mirrored surface 22 falls on thephotocell lib. The voltage drop across resistor ll is then at a maximum.The drop across the plate resistor ll is applied through resistor" inthe screen grid lla of an oscillator tube II which is a 8L operated at afrequency of two megacycles. The circuit for the oscillator ll isconstituted by a tuned grid component which includes inductance El andvariable condenser II, and a tuned plate circuit which comprises a plateinductance 6i and variable condenser 62. Resonance of the grid platecircuit of theoscillator ll is indicated by a flashlightbulb ll which isenergized by a inductance Ii. A coil 66 for heating the mirrored surfaceI! of cap II by induction is. placed over the cap II as shown. The highfrequency magnetic field produced in the coil threads through the cap licausing the flow of eddy currents therein. Since these currents tend toflow on the surface, substantialiy only the mirrored surface portion 12of the cap II is heated. Connected in circuit with cell ll is a radiofrequency ammeter ll, a variable condenser ll, and an inductance llwhich is coupled to the plate inductance ii. The series connectedcondenser I1 is provided for tuning the circuit of the heating coil llto resonance which condition is indicated by maximum deflection of theammeter ll.

The frequency of the heating 'current should be so chosen that theeilective depth of penetration of the eddy currents satisfy a conditionor minimum heat dissipation with accurate thermocouple indication. For acap member Ii of the size described, a frequency of two megacyclesproved quite satisfactory.

OPERATION The bridge network containing photo-cells lla and lib, anddoor ll are adjusted until the milliammeter ii connected across theneutral points 41, ll of the bridge reads zero. Under this condition,the grid-cathode circuits of the amplifiers 42 and ll have the samebias, and hence there will be no difference of potential between the end,terminals of resistor ll.

The two-megacycle oscillator is is next tuned to resonance. Thiscondition will be reached when lamp ll reaches its maximum brilliance.

The load circuit controlling heater coil is then tuned until ammeter llreaches a maximum.

After the above adjustments have been made. the solid CO: or any othersuitable type refrigerant is placed inthe coolant chamber i I, and thewas previously explained that with no condensation on the mirrored face22, the beam of light passes by the edge of photo-cell Ilb along path a.However, when condensation on face 22 begins to take place, the lightbeam from lamp I3 is dispersed upon reflection from the mirrored face22. and some of the light now passes along path b into the photo-celltlb. The bridge circuit including photo-cells 3|a, 3|b and resistors 44,I now becomes unbalanced, and a voltage appears across bridge terminalsl1, It. This voltage is then impressed across the grids Hit-43a of tubes42 and 43, respectively, and changes the drop across load resistor IIdriving grid 56b more negative with respect to cathode a. This, in turn,decreases the current flow in the anode-cathode circuit of tube 55 anddecreases correspondingly the voltage drop across resistor 58. Thisincreases the voltage on the screen grid 58a of oscillator tube 58,thereby increasing the amplitude of the current or oscillation appearingacross the tuned output circuit comprising capacitor 62 and coil 6 This,in turn, causes an increase in the high frequency (2 megacycles)alternating current flowing through coil 65 and increases the heating ofthe mirrored surface 22 until the condensation thereon begins toevaporate. As eva oration progresses, the dispersion of light on themirrored surface 22 also decreases. Less and less light now falls onphoto-cell 3|b, and the bridge circuit is 1 brought more and more into abalanced state.

A reverse effect is then produced on resistors 53 and I6, thusdecreasing the voltage on screen grid Ila of oscillator 58 and hencealso the current flow in coil SI and through heating coil 65. When thetemperature of the surface 22 of cap 2| thereby is so reduced thatcondensation begins to form again, light once again begins to fall onphotocell llb, and the cycle is repeated.

This process of alternate heating and cooling of the surface portion 22is substantially continuous. It is evident that the result is thatmirrored surface 22 is maintained at a point of equilibrium where theamount of heat induced in cap 2| from coil 65 substantially balancestheamount of heat taken away from cap 2| by conduction through rods l6and I! so that the temperature of mirrored surface 22 is maintainedsubstantially at the dew-point of the atmosphere in chamber l2. As themoisture content of this atmosphere changes, the temperature of thecondensation will likewise vary and cause the equilibrium point to shiftand the temperature of the mirrored surface 22 to vary accordingly. Aspreviously explained, thermocouple 23 imbedded in cap 2| immediatelybeneath the mirrored surface 22 measures the temperature thereof and maybe recorded by conventional recording apparatus 26.

The invention may be embodied in other speciflc forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the'payment of any royalties thereon or therefor.

We claim:

1. A dew-point type hygrometer comprising, a mirror, a heat sinktherefor, a heat conducting element extending from said mirror into saidsink, a light source, a photo-cell, said light source being arranged todirect light towards said photocell by reflection from said mirror andin such manner that the amount of light fallingon said photo-cell whensaid mirror is wet differs from that falling on said photo-cell whensaid mirror is dry, a coil extending around said mirror only, means toenergize said coil with high frequency current to thereby heat thesurface of said mirror by induction, and circuit means including saidphoto-cell for controlling operation of said mirror heating means.

2. A dew-point type hygrometer comprising, a mirror, a heat sinktherefor, a heat conducting element extending from said mirror into saidsink, a, light source, a photo-cell, said light source circuit meansincluding said photo-cell for controlling operation of said mirrorheating means.

3. A dew-point type hygrometer comprising, a mirror, a heat sinktherefor, a light source, a first photo-cell, said light source beingarranged to direct light towards said first photo-cell by reflectionfrom said mirror and in such manner that the amount of light falling onsaid photocell when said mirror is wet differs from that falling on saidphoto-cell when said mirror is dry, a coil associated with said mirror,means to energize said coil with high frequency current to thereby heatsaid mirror by induction. a second photo-cell, means connecting saidfirst and second photo-cells in a bridge, and means connecting theoutput of said bridge to control operation of said mirror heating means.

4. A dew-point type hygrometer comprising, a mirror, a heat sinktherefor, a light source, a first photo-cell, said light source beingarranged to direct light towards said photo-cell by reflection from saidmirror and in such manner that,

the amount of light falling on said photo-cell when said mirror is wetdiffers from that when said mirror is dry, a coil associated with saidmirror, means to energize said coil with high frequency current tothereby heat said mirror by induction, a second photo-cell, a bridgecircuit, said bridge circuit including said photo-cells in oppositebranches thereof, and means connecting the output of said bridge circuitto control operation of said mirror heating means.

5. A dew-point, type hygrometer comprising, a mirror, a heat sinktherefor, a light source, a. first photo-cell, said light source beingarranged to direct light towards said photo-cell by reflection from saidmirror and in such manner that the amount of light falling on saidphoto-cell when said mirror is wet differs from that when said mirror isdry, a coil associated with said mirror, means to energize said coilwith high frequency current to thereby heat said mirror by induction, asecond photo-cell, a bridge circuit, said bridge circuit including saidphoto-cells in different branches thereof, means connecting the outputof said bridge circuit to control operation of said mirror heatingmeans, said means direct- 7 ing a relatively small amount 0! light fromsaid source into said second photo-cell to thereby compensate for straylight falling on said first photo-cell from said light source.

6. Dew-point type hygrometer apparatus comprising, a source of light, apairof 12 cells symmetrically mounted with respect to said source oflight, a light shield surrounding said photoelectric cells to shieldthem from direct rays of light from said source of light,- a mirrorpositioned to reflect light from said source of light to one of saidpair of photoelectric cells when moistuneiscondensedonsaidmirrona portin said light shield to control the amount of indirect light received bythe other of said cells from said light source to compensate for theamount of indirectlightreceived by said one photoelectric cell and meansfor diflerentially combining the outputs of said cells so the combinedoutput is zero when no light reflected by said mirror from said lightsource is received by said one photoelectric cell.

7. Dew-point type hygrometer apparatus comprising, a housing having aplurality of chambers, a source of light mounted in one of saidchambers, a pair of photoelectric cells mounted therein, a light shieldsurrounding said photoelectric cells in said chamber to shield them fromdirect rays of light from said light source, a port in said light shieldto admit light reflected from said source to the first photoelectriccell of said pair, a mirror positioned in a second chamber in-saidhousing to reflect light, when moisture is condensed thereon, from saidlight source through said port to said first photoelectric cell, anadjustable port in said light shield to control the amount of indirectillumination received by the second photoelectric cell of said pair fromsaid light source to compensate for the amount of indirect illuminationreceived by said flrst photoelectric cell, said photoelectric cellsconnected in adjacent legs of a Wheatstone bridge circuit the ss,as7 Itoelectric cell to compensate for the amount of indirect illuminationreceived by said one photoelectric cell th'rough said one port with anelectronic circuit, the outputs of said photoelectric cells beingdiflerentially connected therein so that the output of said circuitenergizes said heating means when moisture is condensed on thereflecting surface of said mirror independently of any fluctuation inintensity of light in said light source.

9. In hygrometer apparatus for determining th dew-point of a gascontaining moisture, said apparatus including a mirror, a heat sinkthere for, a heat conducting element extending from said mirror intosaid sink, a light source and a photo-cell, said light source beingarranged to direct light toward said photo-cell by reflection from saidmirror and in such manner that more light from said light source strikesthe photo-cell when said mirror is l ged by moisture condensate thanwhen it is free of said condensate, the combination of a coil woundaround the'body of said mirror in relatively close proximity to thereflecting surface thereof with a high frequency circuit meanscontrolled by the output of said photo-cell whereby said coil isenergized and the surface of said mirror is rapidly heated by inductionand is maintained at a temperature relatively close to the dew-point ofthe moisture in the gas being tested.

10. In hygrometer apparatus for determining the dew-point of a gascontaining moisture, said output from which is zero when no lightreflected by said mirror is received by said first photoelectric celland the amount of indirectly reflected light received by each saidphotoelec: tric cell is equal and compensated.

8. In dew-point type hygnometer apparatus having a source of light, amirror positioned to reflect light therefrom and means for heating saidmirror the combination of a pair of photoelectric cells symmetricallypositioned with respect to said source of light and a shield surroundingsaid pair of photoelectric cells to shield them from direct illuminationfrom said source of light, said shield provided with a pair of ports,one to admit light to one of said photoelectric cells reflected by saidmirror when moisture is condensed on. the reflecting surface thereso ofand the other port to control the amount of indirect illuminationreceived by the other phoapparatus including a mirror, a heat sinktherefor, a heat conducting element eztending from said mirror into saidsink, a light source and a photo-cell, said light source being arrangedto direct light toward said photo-cell by reflection from said mirrorand in such manner that more light from said light source strikes thephotocell when said mirror is f ged by moisture condensate than when itis free of said condensate, the combination of a coil surrounding thebody of said mirror substantially in the plane of the reflecting surfacethereof with a high frequency circuit means controlled by the output ofsaid photo-cell whereby said coil is energized when moisture iscondensed on said reflecting surface and said mirror is rapidly heatedby induction and is maintained at a temperature relatively close to; thedew-point of the moisture in the gas beingtested.

FRANK A. FRISWOLD. RALPH. D. LEWIS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,059,976 Stargardter Nov. 3,1936 2,268,785 Thornthwaite Jan. 6, 1942

